The present invention relates to industrial process control and monitoring systems. More specifically, the present invention relates to diagnostics of industrial process control and monitoring systems which utilize two-wire process control loops to transmit information.
Industrial process control and monitoring systems are used in many applications to control and/or monitor operation of an industrial process. For example, an oil refinery, chemical processing plant, or paper manufacturing facility may have numerous processes which must be monitored and controlled.
In such industrial processes, process variables are measured at remote locations across the process. Example process variables include temperature, pressure, flow and the like. This information is transmitted over a two-wire process control loop to a central location, for example, a control room. Similarly, process variables can be controlled using controllers placed in the process. The controllers receive control information from the two-wire process control loop and responsively control a process variable, for example by opening or closing a valve, heating a process fluid, etc.
Various protocols have been used to communicate on two-wire process control loops. One protocol uses a 4-20 mA signal to carry information on the loop. The 4 mA signal can represent a zero or low value of a process variable while the 20 mA signal can represent a high or full scale value. The current can be controlled by a process variable transmitter to values between 4 and 20 mA to represent intermediate values of the process variable. A more complex communication technique is the HART® communication protocol in which digital information is superimposed onto a 4-20 mA signal. Typically, in such configurations a separate two-wire process control loop is required for each field device.
A more complex communication technique used on two-wire process control loops is generally referred to as fieldbus-based protocols, such as Foundation™ fieldbus. In a Fieldbus protocol, all information is transmitted digitally and the analog current level on the process control loop is not required to carry information. One advantage of such a configuration is that multiple process variable transmitters or controllers can be coupled in series on a single process control loop. Each device on the loop has an address such that it can identify messages which are addressed to it. Similarly, messages transmitted by a field device can include the address of the device so that the sender can be identified.